We've got a force of previously unknown aliens in the swamp.

Timothy tells them that his ship was attacked by some unknown aliens.

Of course I should be the one to answer; you need somebody who's able to negotiate with these unknown aliens.

There is also a section on unknown aliens.

"One of the things I like least about this is sending a civilian into a nest of unknown aliens."

The entire third season of the series centered on this group of previously unknown aliens.

Seven wanted to know more about these unknown aliens, but she let the matter drop for the moment and pursued another subject matter.

Spock glanced again at the screen, looking for clues to how the unknown aliens had thought.

These unknown aliens are waiting for an answer.

Of the unknown alien, and the awful gifts it brought?